Passenger comfort in the cabin space of a commercial aircraft depends to a considerable extent on the prevailing noise level in the cabin space, which is generated by various sound sources on board the commercial aircraft. Thus, the air-conditioning system, which comprises various components of an air distribution ducting system as well as fan devices, creates a noise level that is clearly detectable by the passengers.
The noise generated by the air-conditioning unit is of a flow-acoustic type. Any measure used to absorb or reduce the produced resultant sound involves extra effort and expenditure, since the necessary components for this purpose, such as for example sound absorbers, contribute to the weight and the overall costs of a commercial aircraft. The aim of all attempts to reduce the cabin noise is primarily to control the noise, i.e. eliminate or at least mitigate the flow-acoustic sources in the air conditioning system.
In the air distribution ducting system of the air-conditioning unit diaphragms are used that regulate and throttle the amount of air and the volume flow through the air distribution ducting system. Conventional diaphragms have for example the shape of a metal ring with an external diameter adapted to the internal diameter of the air-conditioning duct, as well as an axially centrally arranged passage that has a smaller diameter compared to the external diameter of the diaphragm. Another conventional diaphragm is designed for example as a multi-aperture diaphragm.
If the volume flow in a pipe encounters an obstacle, such as for example a diaphragm, the fluid particles become detached from the inner wall of the pipe and an eddying of the wake flow occurs downstream of the diaphragm. So-called turbulence bundles, which act on the pipe wall, are formed by the change in the flow conditions after the diaphragm and by the pressure rise generated in the wake downstream of the diaphragm. At the same time, under certain flow conditions, the diaphragm itself can be excited to perform vibrations. Thus, the sound is produced primarily by dipolar sources, i.e. by excitation of the diaphragm itself and by interaction of the turbulence bundles with the pipe wall of the air distribution ducting system. The generated sound propagates through the air-conditioning system and thus reaches the aircraft cabin.
A diaphragm of predetermined geometry can generate only a certain pressure loss, which is dependent on the other flow conditions. In order to supply different amounts of air to the different regions of the air distribution ducting system, such as for example the temperature zones in the cabin, the freight compartment, the avionics, etc., various types of diaphragm are accordingly employed, which depending on their size and shape generate different pressure losses, whereby the volume flow in these regions can be adjusted as desired.
The connection between the pressure loss generated by the diaphragm and the volume flow through the diaphragm can be determined by means of the following equation:
                              Δ          ⁢                                          ⁢          p                =                  ς          ·                      ρ            2                    ·                      U            2                                              (        1        )            where ρ is the density of the flowing medium, U is the volume flow and ç depends on the geometry of the diaphragm and, in the case of an annular diaphragm, is defined as follows:
                    ς        =                              (                                                            A                  1                                                  α                  ·                                      A                    2                                                              -              1                        )                    2                                    (        2        )            
In this connection A1 is equal to the pipe cross-section upstream of the diaphragm, A2 is equal to the pipe cross-section downstream of the diaphragm and α is equal to a constant, which in the case of sharp (acute) diaphragm edges is equal to 0.63.
The continuity equation determines the dependence between the volume flow dV/dt and the flow velocity U. The following relationship exists:
                                          ⅆ            V                                ⅆ            t                          =                  U          ·          A                                    (        3        )            where for a circular pipe of diameter d the cross-sectional surface amounts toA=π·d2/4|
In order to keep the sound level in the aircraft cabin as low as possible, a diaphragm installed in the air distribution ducting system should in the ideal case perform two types of functions. Firstly, it should generate a pressure loss at the installation site in the air-conditioning duct. Secondly, the sound level generated by it should be as low as possible.
Although the pressure loss generated by the diaphragm is not desirable from the acoustic aspect, nevertheless from the flow technology aspect, it is necessary in order to be able to adjust the volume flow and thus the throughput of the volume flow through the air distribution ducting system.
An object of the present invention is accordingly to provide a diaphragm for throttling a volume flow in an air distribution ducting system of an air-conditioning unit of a commercial aircraft, which on the one hand generates a desired pressure loss, but on the other hand generates a low sound level, in order to thereby configure the air-conditioning system of the aircraft so that it generates little noise.